module (03) arc breaking technologies and current interrupt

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LV , MV & HV Switchgear

Module

03Arc Breaking Technologies and Current Interruption

Module(03)ArcBreakingTechnologiesandCurrentInterruption Page1of22

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3.1INTRODUCTION Thecircuitbreakers,astheyhavebeendenedearlierinchapter1,areautomaticswitcheswhichcaninterruptfault currents.Insomeapplications,likesinglephasetractionsystems,singlepolecircuitbreakersareused.Thepartofthe circuitbreaker connected in one phase is called the "pole". The circuitbreaker suitable for three phase system is called a triplepole circuitbreaker. Each pole, of the circuitbreaker, comprises one or more "interrupters" or "arc extinguishingchambers".Theinterrupterenclosesasetof"fixedandmovingcontacts".Themovingcontactscanbe drawnapartbymeansoftheoperatinglinksofanoperatingmechanism. The electric arc, produced by the separation of current carrying contacts, is a type of electric discharge between electrodes. Incircuitbreakers, the arc persists during the briefperiod after separation of currentcarrying contacts. Thecircuitbreakershouldbecapableofextinguishingthearcwithoutgettingdamaged.Thearcisinterruptedbya suitablemediumandbyadoptingsuitabletechniqueforarcextinction.Thecircuitbreakerscanbeclassifiedonthe basisofarcextinctionmedium. Thearcplaysanimportantroleinthebehaviorofthecircuitbreaker.Theinterruptionofd.c.arcsisrelativelymore difficult than a.c. arcs. In a.c. arcs, as the current becomes zero during the regular wave, the arc vanishes and it is prevented from restriking. Circuitbreakers employ various techniques to extinguish the arc resulting from the separation of the current carrying contacts. The mode of arc extinction is either "high resistance interruption" or "zeropoint interruption". High Resistance Interruption: In this process, the resistance of the arc is increased by lengtheningandcoolingittosuchanextentthatthesystemvoltageisnolongerabletomaintainthearcandthearc getsextinguished.Thistechniqueisemployedinairbreakcircuitbreakersandd.c.circuitbreakers. Low Resistance or Zeropoint Interruption: In this process, the arc gets extinguished at natural current zero, of the alternatingcurrentwave,andispreventedfromrestrikingagainbyrapidbuildupofdielectricstrengthofthecontact space.Thisprocessisemployedinalmostalla.c.circuitbreakers.Thetheoryandmannerinwhichthevarioustypesof circuitbreakerandcontactorextinguishthearcandinterruptthecurrentisdealtwithinthefollowing. 3.2BREAKINGTECHNOLOGIESINBRIEF 3.2.1ARCEXTINGUISHERS Anarcextinguisheristhecomponentofthecircuitbreakerthatextinguishesanarcwhenthecontactsare opened.Figure3.1showsarcextinguishersinalowvoltagecircuitbreaker.Anarcisadischargeofelectric currentcrossingagapbetweentwocontacts.Circuitbreakersmustbedesignedtocontrolthembecausearcs cannot be prevented. There are four techniques to extinguish an arc and there are several arc control methods.Inthistopic,thosemethodswillbeintroduced.

Figure(3.1)Thearcextinguishersinalowvoltagecircuitbreaker Module(03)ArcBreakingTechnologiesandCurrentInterruption Page2of22

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3.2.2WHATISANARC? Doyoueverrecallpullingaplugfromawallsocket(Fig.3.2)andseeingwhatappearedtobesparks?What youwereobserving,onaverysmallscale,wasanattemptatarcformationbetweenthewallcontactsand theplugcontactsinyourhand.Forthesakeofthisdiscussion,letsdefineanarcasadischargeofelectric currentcrossingagapbetweentwocontacts.

Figure(3.2)TypicalARC Arcsareformedwhenthecontactsofacircuitbreakerareopenedunderaload.Arcscanbeverydestructiveandvary greatly in size and intensity. The size of the arc depends on the amount of current present when the contacts are pulledapart.Forexample,anarcthatformswhennormalloadcurrentisbrokenisinsignificantcomparedtothearc that forms when a short circuit is broken. Because arcs cannot be prevented, circuit breakers must be designed to controlthem. Theheatassociatedwithanarccreatesanionizedgasenvironment.Themoreionization,thebettertheconditions are for an arc to be maintained and grow. The bigger the arc, the more heat created, which increases ionization. Arcingisaconditionthatmustbedealtwithquicklyandeffectivelybyacircuitbreaker. Theimportantthingtorememberhereisthattheabilityofthecircuitbreakertocontrolthearcisthekeytoitsshort circuit interrupting capability. This is a critical factor for selecting circuit breakers. A short circuit is the most devastatingovercurrentcondition. CurrentZeroorZeroPointisaveryimportantaspecttoarcextinguishing.Atcurrentzero,conditionsareoptimalfor preventinganarcfromcontinuing.ThecurrentissaidtobeCurrentZerowhenthesinecurveisat0,180and360 (Fig.3.3).

Figure(3.3)CurrentZero Voltage is also a very important consideration because it is the pressure that keeps the current moving. Left unchecked,voltagewillkeeppushingthecurrentthroughcurrentzeroandgivenewlifetothearc.Voltagedoesnot takekindlytobeingstoppedinitstracksduringtheextinguishingofanarc.Ifitreignites,itcandamagethewhole electricalsystem. Module(03)ArcBreakingTechnologiesandCurrentInterruption Page3of22

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Circuitbreakerstakethisprocessintoaccountbysimultaneouslyopeningthecontactsandextinguishingthearc.The successfulextinguishingofthearcdependsontheDielectricStrengthofthegapbetweenthecontacts.Thedielectric strength is the maximum voltage a dielectric can withstand without breaking down. A Dielectric is any insulating material between two conductors. In these discussions, the circuit breaker contacts are the conductors and the insulatingmaterialcanbeair,gasoravacuum.Ifthedielectricstrengthisgreaterthanthevoltagetryingtoreignite thearc,thearcextinguishingwillbesuccessful. The invention of a device called DEION arc extinguisher in the early 1900s by Westinghouse was a revolutionary advanceinarcinterruption.Improvedversionswereusedforyearswithamajorityofcircuitbreakersandcontinueto beusedtodaywithlowvoltagecircuitbreakers. Anumberofotherapproachestoarccontrolhavealsobeentried.OneofthesuccessstoriesistheuseofVacuum Interruptionwithmediumvoltagebreakers. 3.2.3ARCCONTROLTECHNIQUES Eachapproachhasmadeimprovementstoitsinitialconceptinanefforttoextinguisharcsmoreefficiently.Arc controlmethodsutilizeoneormoreofthefollowinggeneraltechniques: StretchingArc:Thearcisproducedwhenthecontactspart.Asthegapwidens,thearcisstretched(Fig.3.4) andcooledtothepointwhereitisextinguished.

Figure(3.4)StretchingARC BreakingArcintoSmallerPieces:Thearcisproducedwhenthecontactspart.Thearcmovesupintothearc dividerandsplits(Fig.3.5),coolsandisextinguished.

Figure(3.5)BreakingARC BlowingOutArc:Inthismethod,ahighpressuregasblowsthearcintoanarcdividertobeextinguished(Fig.3.6).

Figure(3.6)Blowingoutarc Module(03)ArcBreakingTechnologiesandCurrentInterruption Page4of22

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Enclosing Contacts: In this method, the contacts are housed in an oxygenfree enclosure (Fig. 3.7) with a dielectric suchasavacuum,gasorcoolingoil.Withoutoxygen,thearccannotsustainitselfandthearcisextinguished.

Figure(3.7)Enclosingcontacts 3.2.4ARCCONTROLMETHODS Therearesixmethodsusedaroundtheworldtodaytodealwitharccontrol,arcchute,vacuuminterrupter,SF6 minimumoil,magneticcoil,Puffer The arc chute method: only uses the Breaking Arc into Smaller Pieces technique. Arc chutes are normally associated with low voltage circuit breakers due to efficiency and cost. In general, an arc chute will confine, divideandcoolanarc(Fig.3.8),resultinginthearcbeingunabletosustainitself.Thereisonearcchutefor eachsetofcontacts.

Figure(3.8)Arcchutemethod Thevacuuminterrupter:methodusestheEnclosingContactstechniquetoextinguisharcs.Thevacuumenables thecontactstobesmallerandeliminatesthedivider,makingthismethodthemostcosteffectiveandefficient above1000V.Arcingtakesplacewithinasealedevacuatedenclosure(Fig.3.9).Thecontactsarelocatedinside andarcingoccurswhenthecontactsareseparated.Becausetheenvironmentinsidetheinterrupterenvelope isavacuum,anarccannotbeeasilysustained.Itwillnotreachtheintensitypossiblewithanarcchute.One vacuuminterrupterisprovidedforeachsetofcontacts.

Figure(3.9)Vacuuminterruptermethod Module(03)ArcBreakingTechnologiesandCurrentInterruption Page5of22

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TheSF6method:alsousestheEnclosingContactstechnique.Itwasaprecursortothevacuuminterrupterand used SF6 gas as the dielectric (Fig. 3.10). The heat energy created by the arc works to break apart the SF6 molecules.Thelargerthearc,thegreaterthebreakdownofthegaswhichaidsinextinguishingthearc.

Figure(3.10)SF6method Theminimumoilmethod:alsousesEnclosingContactswithoilasthedielectric(Fig.3.11).Thearcenergyis absorbedasitripshydrogenawayfromtheoilmolecule.Theoilitselfalsohelpstocoolthearc.Ascurrentzero isapproached,moreoilisdrawnintothesystem,furthercoolingandDeionizingthearc.Itisusedtodayinlow voltagesituationsandpotentiallyexplosiveenvironmentswhereanarcchuteisnotdesirable.

Figure(3.11)Minimumoilmethod The magnetic coil method: uses the Breaking Arc into Smaller Pieces technique. It is very similar to the arc chutemethod.Thenaturalmovementofanarcisupward,inthisinstance,intoanarcchute.Acoil,calleda blowout coil, is located in the centre of the arc chute (Fig. 3.12). The arc is broken into two. The arcs are lengthened and cooled as they rise higher. The cooling reduces the rate of ionization. When the ionization dropsbelowthelevelnecessarytosustainthearcs,theyextinguishatcurrentzero.Priortovacuuminterrupter technology becoming the method of choice with medium voltage power breakers for extinguishing arcs, the magneticcoilmethodservedwellformanyyears.

Module(03)ArcBreakingTechnologiesandCurrentInterruption

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Arcrunners Arcsplitters Elongatedarc Arcinprocessoftravelling Blowoutcoils Originofarc Figure(3.12)Magneticblowoutcoil The puffer method: uses the Blowing Out Arc and Enclosing Contacts techniqu